This invention relates to linear actuators, and in particular, to a linear actuator having a slip clutch structure arrangement which eliminates the need for electro/mechanical limit switches. While the invention is described in particular detail with respect to linear actuators, those skilled in the art will recognize the wider applicability of the described invention.
Linear actuators are well known in the art. They are typically connected to gear motors to convert the rotational output of the gear motor into linear motion to move an object linearly. Linear actuators find use in devices such as hospital beds to raise and lower the head and foot of the bed, in chairs, to raise and lower the chair, and a number of other similar and dissimilar devices.
Linear actuators typically include a tube to which the device is connected and a shaft which is threadably received in the tube. This shaft is operatively connected to the motor so as to be rotatively driven thereby. As the motor rotates the shaft, the tube moves forward or backward on the shaft, depending on the direction of rotation of the motor. The actuator assembly usually includes limit switches which limit the number of rotations the motor or the distance of travel this tube may make in any one direction. When the limit is reached, the limit switch cuts off power to the motor preventing further movement of the tube. The motor can then only operate in the opposite direction.
The limit switch is generally part of an assembly which includes a plurality of gears to reduce the speed of the output so that the limit switch will properly control the number of rotations. Such an assembly is shown, for example, in U.S. Pat. No. 5,063,808, assigned to the assignee of the present invention and incorporated herein by reference. For the control to work properly, the gears must be properly aligned with each other, and the ratio between the gears must be properly determined. If the gears are not properly aligned, or the gear ratios are not properly determined, the timing of the limit switch will be off and the limit switch will turn off the motor too early or too late, causing the linear actuator to extend too far or not far enough. Because of the precision required for the gearing, the limit switch assemblies can be relatively complex, and hence expensive to produce.